The Scanning Transmission Electron Microscope Resource at Brookhaven has been established in order to provide for the efficient use of expensive instrumentation essential for progress in biological structure studies. An instrument similar to that introduced by Dr. A. V. Crewe has been constructed in the Biology Department of Brookhaven National Laboratory. The instrument is capable of producing a 2.5 A probe and using this to image single heavy atoms in frozen specimens. The instrument is also capable of quantitative electron microscopy of unstained biological specimens, giving total molecular weight of single molecules and mass distribution within a molecule. The imaging mode employed, annular detector, produces an image without phase contrast artifacts and with minimum possible dose to the specimen. The microscope has been available to users since November 1977. An Advisory Committee composed of Brookhaven biologists and academic biologists has been formed to oversee the application of the microscope to the study of a variety of biological materials on a time-sharing basis. This application includes a description of progress to date on the structure of nucleosomes, fibrinogen, heavy atom stained filamentous virus fd, meningo polysaccharides and cyanobacterial phycobilisomes, as well as studies of atom motion at low temperature, heavy atom visibility and development of new negative stains. Research proposals in less explored areas which are expected to expedite the application of the microscope for biomedical research are presented. The facility will continue to provide for experimental work in sample preparation, heavy atom labeling, image analysis and the possibility of new instrumental developments for electron microscopy. Education and training of scientists in the potentials and techniques of electron microscopy will continue to be emphasized. Access to the facility will continue to be open to all qualified biomedical scientists and will be regulated by the STEM Advisory Committee to promote its efficient functioning as a national resource in health related research and training.